(1) Field of the Invention
The present invention relates to a fixing device for use in a printer, a facsimile device, a copier or the like, and to an image forming device provided with the fixing device, and especially to a technology for preventing generation of gloss difference on toner images fixed by a fixing member.
(2) Related Art
In recent years, among fixing devices for use in an image forming device such as a printer, a facsimile device, and a copier, more and more fixing devices have been made to thermally fix a toner image onto a recording sheet via a belt having small heat capacity for energy saving.
For example, Japanese Patent Application Publication No. 2007-17495 (Patent Document 1) discloses a fixing device which is configured to make a recording sheet, on which a toner image has been transferred, pass through a fixing nip to thermally fix the toner image onto the recording sheet, where the fixing nip is formed by causing a pressing roller to press a fixing roller via an endless fixing belt that is driven to perform a circulating motion.
In such a fixing device, when the recording sheet passes through the fixing nip area, heat is absorbed by the recording sheet from a part of the fixing belt that contacts the recording sheet. As a result of this, in the fixing belt, a difference in temperature is generated between the part that contacted the recording sheet and the other part.
Furthermore, when the succeeding recording sheet is subjected to the thermal fixing while the succeeding recording sheet is on the part of the fixing belt that contacted the previous recording sheet and lost heat, the fixing temperature for the succeeding recording sheet is lower than the fixing temperature for the previous recording sheet, resulting in a difference in gloss level (gloss difference) between the fixed toner images on the previous and succeeding recording sheets.
Also, when the recording sheet is long in the direction in which the recording sheet passes through the fixing nip area, there will be a gloss difference between parts of the same recording sheet. The gloss level of the toner image varies depending on the fixing temperature. Therefore, the larger the rate of decrease in the fixing temperature is, the larger the gloss difference is. In the above-described fixing device, to lessen the gloss difference, the recording sheet is pre-heated by the heater before it is transported to the fixing nip area.
With the pre-heating, the difference in temperature between the recording sheet and the fixing belt is reduced, and thus the amount of heat to be absorbed by the recording sheet from the fixing belt is reduced. As a result of this, the difference in temperature between the part of the fixing belt from which the heat is absorbed by the recording sheet and the other part is reduced. This makes it possible to lessen the gloss difference that is generated between different recording sheets or between different parts of a same recording sheet.
However, when, as in the technology disclosed in Patent Document 1, the recording sheet is pre-heated by the heater, the temperature inside the image forming device is increased, and the developing unit located near the heater is also heated. This produces a problem that the toner stored in the developing unit is apt to become hard by the influence of the heat, having an adverse effect on the image forming operation.